Compared to a battery, an energy harvesting (EH) power source, a type of electrical power source, has fundamentally different energy- and power-related behavior. A battery has a finite amount of energy stored in it, but can (in principle), source an arbitrarily high peak power—limited only by the non-idealities of the cell (e.g., its internal resistance). An EH source, on the other hand, can source energy for an indefinitely long time, but has a finite peak power capability. Compared to a load powered by a battery, a load powered by an EH source therefore has fundamentally different requirements placed on it.
A load powered by a battery must be energy-efficient. Since the battery's stored energy is limited, while its peak power sourcing ability is (substantially) not limited, the relevant performance metric of the load is the energy consumed per unit of work performed. A well-designed load therefore operates in burst mode, using a low duty cycle to maximize sleep time and, therefore, minimize the amount of energy consumed per activity performed. Its peak power consumption will be much higher than that of the equivalent EH-sourced design.
A load powered by an EH, conversely, must be power-efficient. Since the peak power sourcing ability of the EH is limited, while the amount of energy it can source is indefinitely large, the relevant performance metric of the load is its peak power consumption. A well-designed load therefore operates in a continuous mode, eschewing sleep modes and, therefore, minimizing its peak power consumption. Its energy consumption will be much higher than that of the equivalent battery-sourced design.
Given the foregoing, it can be appreciated that an optimally-designed load powered by an EH power source, therefore, behaves quite differently from an optimally-designed load powered by a battery.